17.1 Blood

Hematopoiesis

Hematopoiesis is the formation and development of blood cells. Today one assumes that all the cells of the hematopoietic system stem from a stem cell line. The first of such stem cell lines forms in the mesoblast (extraembryonic mesoderm) of the umbilical vesicle (stage 7, ca. 19 days, 7). Recent investigations (1, 2) have shown that definitive stem cell lines for hematopoiesis probably come from a center within the embryo. This is located in the splanchnopleura around the dorsal aorta at the level of the navel.

Fig. 1Intra- and extraembryonic blood islands have formed in this embryo, whereby the definitive cells of the stem cell lines stem from a region near the dorsal aorta.

Blood and vessel formation therefore first occurs extraembryonally on the umbilical vesicle and in the chorionic villi (extraembryonic phase of blood and vessel formation). Subsequently follows the intraembryonic phase, which is subdivided into hepatolienal and myeloid phases. First vessels and pluripotent stem cells arise within the embryo. These migrate from the dorsal aorta near the navel and mainly colonize the liver (hepatic phase). A small portion - primarily for erythropoiesis - appears after the 12th week in the spleen (lienal phase).
In the myeloid phase daughter cells of these stem cells emigrate via the vessel system into the bone marrow and increasingly take over the formation of the blood. They are lifelong able to renew and differentiate themselves; they remain in the bone marrow and, according to need, can differentiate into every type of lymphoid and also myeloid cells.

The surface antigen CD34 carrying cells, which are the first precursor cells for the hematopoietic system, can be detected in the ventral wall of the dorsal aorta with immunhistochemical markers. Out of them arise both endothelial cells and also hematopoietic stem cells that are responsible for the formation of the vascular system as well as for the lifelong production of blood cells. Hematopoietic stem cells form cell clusters when they are stimulated by cytokine andvariousgrowth factors.